The present invention relates to an optical film manufacturing method and an optical film manufacturing method for manufacturing an optical film used in various forms of display apparatuses such as a liquid crystal display apparatus (LCD) or organic electroluminescent display, particularly to an optical film having a birefringence and used for a polarizing plate protecting film used in these display apparatuses and a retardation film.
Generally, a liquid crystal display apparatus is basically arranged in such a way that a polarizing plate is provided on both sides of the liquid crystal cell. The polarizing plate permits the passage of only the light on the plane of polarization in a predetermined direction. It plays a major role in the liquid crystal display apparatus in the sense of making visible a change in the light distribution of the liquid crystal due to an electric field. The performances of the liquid crystal display apparatus greatly depends on the performance of the polarizing plate.
In recent years, there has been an increasingly stringent requirement for higher display quality of a thin-film liquid crystal display apparatus. To meet this requirement, various forms of liquid crystal display methods such as VA, OCB and IPS have been proposed. To provide a wide angle of visibility, a retardation correction film is commonly employed in a liquid crystal display apparatus. A retardation film is required to provide higher and higher quality to meet the demands for a larger screen size and higher definition. This makes it necessary to satisfy the requirements for widthwise and lengthwise uniformity in film retardation values.
In addition to the absolute value for the film retardation, there is also a stringent requirement for the retardation in the direction (orientation angle) of slow phase axis (orientation axis) particularly in the retardation film having a larger in-plane retardation. An accuracy within ±1°, preferably within ±0.3° through within ±0.5° must be met over the entire film range.
In the conventional optical film production process, a dope solution made of a thermoplastic resin film material dissolved in a solvent is cast onto the rotating metal-made endless belt or a drum (support member) from a casting die, and the solvent is evaporated on the support member so that a dope film (web) is formed. After that, the web is separated by a delamination roll, and is conveyed by a conveyance roll. Then the web is dried in a drying zone, whereby an optical film is produced, according to the conventional art.
As shown in FIGS. 9 and 10, a general drying and spraying apparatus in the conventional drying zone is designed in such a way that hot air is sprayed approximately uniformly to the film 21 along the lateral direction from the tip end slit on the film side of the header chamber 22 which expands along the lateral direction of the film 21 to be conveyed, and which is connected with a hot air supply tube 23. The film 21 is dried in the process.
In the aforementioned optical film production process, particular attention is paid to ensure that the machine accuracy of the conveyance line, the temperature/air volume in the heating/drying facility, and distribution of the film thickness at the time of casting are kept uniform across the film whenever possible. However, there is a limit to the uniformity of production equipment. The required accuracy in the aforementioned retardation value/orientation angle is close to the limit of the machine accuracy. This makes it very difficult to meet the aforementioned requirements.
Further, a long-term maintenance of high accuracy involves further difficulties because of lack of uniformity on the right and left of the production line resulting from the repeated thermal strain applied to the optical film production facility and wear of the sliding portion, including chronological deterioration factors.
Then, a method of making a film thickness distribution uniform has be known in which a film thickness distribution right before a winding process is measured and a width of a slit of a casting die is adjusted based on the measurement result so as to make a film thickness distribution uniform. However, as a requested accuracy for a retardation value is increased, it become difficult to satisfy the requested accuracy with the above conventional method.
Further, even if a film thickness is uniform tight before a winding process, there may be a case that a film thickness on a process before the winding process is not uniform. Because, when the film passes a conveyance line being uneven between right and left sides, the uneven film thickness may be corrected by being applied with uneven forces by the uneven conveyance line.
Further, when a film (web) produced by the solution-casting film-forming method is stretched on the in-line basis to produce an optical film, the film (web) to be conveyed is softened by inclusion of a solvent. Accordingly, the film is heavily affected by lack of uniformity of the conveyance line on the right and left, and is easily subjected to widthwise distribution of retardation value/orientation angle. Especially, in the case that an amount of remaining solvent is greater, unevenness between right and left sides on the conveyance line affects greatly on an optical property of a film. Further, the film having been separated from the support member is subjected to widthwise distribution of optical characteristics due to irregularity in film thickness and widthwise inconsistency in drying. As a result, even if a film thickness in a final product is uniform, a distribution of an optical characteristics along a lateral direction may lowered less than a required accuracy. This widthwise distribution is conspicuous when the film formation speed is increased to improve the optical film productivity in particular.
The following Patent Documents have been disclosed to show the art of producing optical films by the solution-casting film-forming method.
[Patent Document 1] Japanese Non-Examined Patent Publication 2004-145062
The Patent Document 1 is a retardation film manufacturing method containing the step of forming polycarbonate into a cast film by the solution-casting film-forming method and the step of drawing the aforementioned unoriented cast film (unoriented film) in the direction (lateral) perpendicular to the casting direction (longitudinal). According to the method of manufacturing a retardation film disclosed in the Patent Document 1, an unoriented film or longitudinal uniaxial oriented film wherein the variation of the in-plane film thickness does not exceed 5 μm and the residual amount of solvent is 1 through 5 wt % is oriented in the lateral direction. This Patent Document 1 is intended to reduce the variation in the film thickness, thereby ensuring uniform film retardation values.
[Patent Document 2] Japanese Non-Examined Patent Publication 2002-293956
The Patent Document 2 discloses a transparent film wherein the retardation value at the center of the film is defined along the lateral direction of the transparent film made of an amorphous thermoplastic resin; at the same time, (1) the absolute value for the difference between the retardation value at the center of the film and that on the end of the film is defined; (2) the absolute value for the inclination of the retardation from the film end to the film center along the lateral direction is defined, or (3) the standard deviation of retardation along the lateral direction is defined. The Patent Document 2 is designed to reduce the variation in the film retardation.
[Patent Document 3] Japanese Non-Examined Patent Publication 1999-64632
Patent Document 3 discloses a method of manufacturing the retardation film formed by orienting the thermoplastic resin film formed by the solution cast method, wherein the film having been formed is subjected to primary orientation at a predetermined temperature, and is then subjected to secondary orientation at a predetermined lower temperature, whereby a retardation film is produced. This Patent Document 3 is intended to reduce the variation of the film retardation.
The aforementioned Patent Documents 1 through 3 disclose the method of manufacturing the optical film characterized by a smaller variation in retardation. These Patent Documents show the method of reducing the variation in retardation of the optical film wherein a basic facility and/or uniform film thickness is assumed. These techniques have a problem of solving the problem when there is a fine variation in the film thickness or when film forming equipment has widthwise lack of uniformity.